1. Field of the Invention
The present invention relates to a network system, a switch, and a server. More particularly, the present invention relates to a network system comprising a plurality of terminals, a plurality of switches which interconnects those terminals and select correct packet destinations, and a server for controlling those switches. The present invention further relates to a switch and server for use in the above network system.
2. Description of the Related Art
In the context of effective use of limited bandwidth resources in a local area network (LAN), confining the broadcast domain, i.e., the area within which broadcast frames can reach, is one of the promising approaches. Virtual LAN (abbreviated as “VLAN” where appropriate) is known as an enabling technology for this, which allows logical grouping of user terminal stations regardless of their physical locations and connections.
FIG. 20 shows an example of a conventional network system which supports virtual LAN configurations. This system involves a plurality of terminals T1 to T6, which are represented as white and black circles in this FIG. 20. Actually, those terminals are divided into two logical groups, each forming a separate virtual LAN environment; the black-circle terminals belong to a first VLAN, while the white-circle terminals to a second VLAN. Four multiple-port switches SW1 to SW4 are deployed to interconnect the terminals T1 to T6, in each of which appropriate internal connections are provided between the ports for transit switching of transmission frames. Receiving frames from one port, the switches SW1 to SW4 retransmit them to other relevant ports after adding an appropriate tag to each frame in order to indicate to which VLAN the source terminal belongs. When such a tagged frame has reached the last leg of its route, the switch removes the tag from the frame before sending it out to the destination. The network system further comprises a server SV whose services are accessible to the terminals T1 to T6 through the switches SW1 to SW4.
The above conventional network system operates as follows. Suppose here that the terminal T1 is attempting to send a broadcast frame FR. FIG. 21 shows how this frame is transported. The broadcast frame FR transmitted from the terminal T1 first reaches the port P11 of the switch SW2. In this switch SW2, the frame FR is added a tag TG indicating that the source terminal T1 belongs to the second VLAN. Frames in which a tag field has been added are called “tagged frames.” The switch SW2 then retransmits this tagged frame FR toward the next switch SW1 through its port P10.
FIG. 22 provides the detailed structure of an Ethernet frame with a tag field d2. As seen from FIG. 22, the tag field d2 is interposed between MAC header d1 and IP header d3. It is actually composed of the following three fields: User_Priority field d21, Canonical Format Identifier (CFI) field d22, and VLAN Identifier (VID) field a d23. The VID information enables a switch to forward a received tagged frame to the correct direction. That is, the switch determines which port should be used to output the frame, referring to the content of its VID field d23.
Returning to the context of FIGS. 20 and 21, the switch SW1 now receives the frame FR through its port P21. The switch SW1 then determines which port should be used to output the frame FR, referring to its tag field TG. Notice that, in the example system, the terminal T4 coupled to the switch SW3 is a member of the second VLAN and there are no other receiving terminals belonging to that group. Accordingly, the switch SW1 chooses its port P22 to send out the broadcast frame FR (see FIG. 21). Now the switch SW3 receives this frame FR at its port P30 and chooses a relevant output port by examining the tag field TG again. In the present case, the switch SW3 retransmits the frame FR through the port P31 after removing the tag field TG therefrom (see the last leg of the route shown in FIG. 21). In this way, the broadcast frame FR originating from the terminal T1 is delivered to the terminal T4, which is the only receiving member of the second VLAN.
There is a standardized format of the tag field (IEEE 802.1Q) for the purpose of VLAN implementations, and switches conforming to the standard specifications are increasingly common in the market today. As opposed to this increasing prevalence of standardized switch products, few terminal stations (including network interface cards for them) support the standard specifications for VLAN. To construct VLANs, it is therefore necessary to define VLAN membership for each port of each switch. This is, however, a troublesome task particularly in such a network where the terminals are administrated under an IP address based management policy. That is, the network administrator has to manually change the VLAN membership definitions of a relevant switch, every time a terminal station is moved, added, or removed.